NSFree Identifying Strings

Hunt Hypothesis

Adversaries may use NSFree identifying strings to exfiltrate data or execute malicious payloads, leveraging compromised credentials for unauthorized access. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect potential data exfiltration or lateral movement attempts early.

YARA Rule

rule NSFreeStrings : NSFree Family
{
    meta:
        description = "NSFree Identifying Strings"
        author = "Seth Hardy"
        last_modified = "2014-06-24"
        
    strings:
        $ = "\\MicNS\\" nocase
        $ = "NSFreeDll" wide ascii
        // xor 0x58 dos stub
        $ = { 0c 30 31 2b 78 28 2a 37 3f 2a 39 35 78 3b 39 36 36 37 }
        
    condition:
       any of them
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints
• Source Rule

False Positive Guidance

• Scenario: A system administrator is using Nslookup to resolve DNS records during routine network troubleshooting.
  Filter/Exclusion: Exclude processes associated with nslookup.exe or any process with nslookup in the command line.

• Scenario: A scheduled job runs NSFree as part of a legitimate software update or patching process.
  Filter/Exclusion: Exclude processes initiated by the task scheduler with known update or maintenance job names.

• Scenario: A developer is using Notepad++ to edit a configuration file that contains the string “NSFree” as part of a placeholder or comment.
  Filter/Exclusion: Exclude processes running notepad++.exe or any process with a known development tool name.

• Scenario: A security analyst is using Wireshark to capture and analyze network traffic that includes the string “NSFree” as part of a protocol payload.
  Filter/Exclusion: Exclude processes running wireshark.exe or any process with a known packet analysis tool name.

• Scenario: A backup job is running Robocopy to copy files that contain the string “NSFree” in their names or contents.
  Filter/Exclusion: Exclude processes running robocopy.exe or any process associated with backup or file transfer tasks.